The present invention relates generally to apparatus and methods for measuring vibration in rotating turbine blades. More particularly, the invention relates to a turbine blade vibration monitoring system employing microwaves.
In the design and operation of a turbine, care must be taken to ensure that no turbine blades are set into vibration during rotation of the one or more blade rows commonly employed. Vibration of a blade sets up fluctuating stresses that can damage or severely reduce the life of the blade. Various means exist to measure these fluctuating stresses while the turbine is in operation. For example, in one arrangement, strain gauges are bonded to the turbine blades and signals from the gauges are transmitted to data acquisition and recording equipment by means of telemetry techniques. This arrangement presents problems with respect to the life of the strain gauges, the number of strain gauges that may be employed, and the cost of the telemetry channels.
Magnetic reluctance (variable reluctance) and eddy current sensors have also been employed to monitor turbine blade vibration. Briefly, reluctance sensors employ a magnet and a coil to detect changes in flux through the magnet when a blade passes and reduces the resistance (reluctance) to the flux. Eddy current sensors employ a coil that is placed in oscillation (resonance) at about 1.6 MHz. When a blade passes, the oscillating field sets up a current (eddy) in the blade metal, which then induces an oscillating field that is picked up by the coil. However, these techniques pose problems.
With regard to bandwidth, the blade vibration monitor (BVM) sensor must support the time resolution of the BVM zero crossing board. (The zero crossing board contains a circuit that outputs a pulse for each blade passage.) For example, one presently available zero crossing board operates at 24 MHz. This bandwidth is expected to increase to 100 MHz with the use of gate array technology. The eddy current sensor operates with a search frequency (i.e., oscillating frequency of the coil) of 1.6 MHz. This is theoretically insufficient for even the 24 MHz zero crossing board. However, the eddy current sensor has been used to support field tests on titanium blades and on steel blades where high residual magnetism has rendered the variable reluctance sensor completely ineffective. The reason the eddy current sensor works at all is believed to be due to the random occurrence of the blade pass pulses with respect to the zero crossing board clock.
Blade capture depth refers to the distance between the BVM sensor and the blade tips. This distance must be small enough to produce a strong signal but large enough so that the blades do not hit the sensor during startup, when the blades grow quickly due to thermal expansion. The variable reluctance sensor will operate at a blade capture depth of 0.200 inch (0.5 cm). The actual gap between sensor and blade tip might be 0.150 inch (0.4 cm) to accommodate variations in blade length with speed and temperature. This is satisfactory for some applications. However, to achieve greater design flexibility, it would be advantageous to provide greater working distance. The eddy current sensor blade capture depth is 0.125 inch (0.3 cm) and is currently used with a blade gap of 0.100 inch. This may present problems due to differential blade expansion during turbine start-up. The eddy current sensor is also sensitive to profile tipped blades, which produce a disastrous double pulse.
In addition, a very hard electrically non-conductive ceramic shield is required around the eddy current sensor to protect it from erosion. Such shields are brittle and difficult to install.
A known millimeter wave radar system measures turbine blade vibration by directing a narrow beam of millimeter wave pulses toward the rotating blade row and analyzing the pulses reflected from the blades to detect any abnormal vibration. U.S. Pat. No. 4,413,519, Nov. 8, 1983, titled "Turbine Blade Vibration Detection Apparatus," discloses this system. The disclosed system is depicted in FIGS. 1-3 attached hereto. Referring to FIG. 1, a plurality of radar sensors S are utilized for determining blade vibration in one or more rotating blade rows. The sensors S are mounted on the casing of a low pressure steam turbine 2 coupled to a generator 4. Two sensors mounted on the outer casing 6 direct their respective radar signals toward the last blade row 8 on the generator end, and two other sensors direct their signals toward the last blade row 8' on the throttle or governor end. Two other sensors direct radar signals toward respective inner blade rows.
FIG. 2 depicts the disclosed mounting for a radar sensor. As shown, the electronics portion 10 of a sensor is mounted on a bracket 12 secured to the outer casing 6. A waveguide 14 carrying electromagnetic energy extends from the electronics to a position adjacent the blade row 8 and in so doing passes through an aperture in the outer casing 6, with the aperture maintaining pressure integrity with a sealing arrangement 18. Within the turbine, the waveguide 14 is enclosed within a waveguide support 20, which in turn is connected to an adjustable support 22 connected to an internal defuser 24.
FIG. 3 is a block diagram of the signal processing circuitry disclosed by the '519 patent. The illustrated circuitry is for two different blade rows examined with two radar sensors per blade row. A first blade counting circuit 30 provides running blade counts on two output lines for two sensor. Another blade counting circuit 31 performs a similar function for the other blade row. Gating circuits G are provided for gating the blade counts and radar signals. A computer provides enabling signals to the gating circuits and analyzes the digitized radar signals. In particular, the computer performs a Fast Fourier transform on the radar signals to obtain a frequency spectrum for each blade.
While overcoming some of the disadvantages of the magnetic reluctance and eddy current techniques described above, the millimeter wave radar system presents problems of its own. For example, the electronic circuitry for transmitting and receiving radar pulses and then analyzing the received pulses to detect vibrations is complex, requiring a mixing of the transmitted and received microwaves to produce a beat containing the velocity of the blades, which must then be removed. The effects of such complexity include reduced reliability, a need for frequent calibration, and high cost. Accordingly, a primary goal of the present invention is to provide a blade vibration monitoring system which, like the millimeter wave radar system, avoids the disadvantages of the magnetic reluctance and eddy current techniques but also has improved accuracy and avoids the complexity and high cost associated with the millimeter wave radar system.